The fabrication of solid-state devices like integrated circuits or image sensors generally proceeds from a microscopically smooth semiconductive wafer through a series of processing steps, such as oxide growth, ion implantation, metal deposition, passivation, etc. Some of these processing steps involve patternwise definition of materials, either during deposition or by pattern-selective removal. Due to such patternwise definition of materials throughout the fabrication sequence, a finished wafer containing a plurality of integrated circuits or image sensors is no longer microscopically smooth, but has topographic features of varying height or depth and of varying lateral dimensions.
When a film is deposited by spin-coating from a liquid precursor on a wafer with such topographic features, the film may exhibit thickness variations extending radially outwardly from major topographic features such as bond pads or test structures. Such radially-directed thickness variations will be referred to as radial streaks.
For a given topographic feature, defined by its lateral dimensions and its height or depth relative to a mean level, the severity of radial streaks originating from said topographic feature increases as the intended thickness of the spin-coated film decreases. For example, few or no radial streaks are generally observed in a film coated on a support with topographic features if the film thickness is 10x of the vertical dimension of the topographic feature. Such a film is said to be a planarizing film. However, if the intended film thickness is 0.1x of the vertical dimension of the topographic feature, the topographic feature functions as a streak-inducing feature and severe radial streaks are often observed in that film. Major topographic features of integrated circuits or image sensors can have vertical dimensions ranging from about 0.2 micrometers to about 10 micrometers. Thus, it will be appreciated that a film coated at 0.25 micrometers intended thickness over a 0.75 micrometer vertical feature can exhibit similarly severe radial streaks as a film coated at 3 micrometers intended thickness over a 9 micrometer vertical feature.
When the film, spin-coated from a liquid precursor, has designated regions to be permanently retained over portions of the integrated circuit or image sensor as an optical or dielectric element, radial streaks in the film can render the device useless.